Although a cell membrane holds the cell together it lets substances in and
out, but these substances must be dissolved in water in order pass to and fro through the cell membrane by
diffusion.
You can think of a partially permeable membrane (*) as having
tiny molecular sized holes in it, that only allow certain small, but NOT
large, particles through.
Particles can be ions (Na+, Ca2+, K+, Cl-) or molecules
(H2O, C6H12O6).
(*)
other phrases used are semi-permeable membrane or selectively
permeable membrane
However, only small molecules and ions can diffuse through the cell membrane
e.g. relatively small molecules like glucose and oxygen for respiration, waste carbon dioxide from
respiration, urea waste, amino acids for protein synthesis and of course water itself, as well as
being the solvent.
BUT big molecules cannot get through the cell membrane e.g. starch and proteins.
In the particle model of a cell membrane on the right, the thick black dotted
line represents the membrane.
Think of the grey circles as the larger molecules like
proteins or starch which cannot pass from left to right through the cell
membrane.
Imagine the blue circles are water - they can pass through
the membrane in any direction - BUT, the net transfer is from a
region
of higher concentration to an area of lower concentration - in this case
from right to left.
Imagine the green circles are small molecules or ions - they might
be able to pass through the cell membrane in either direction, but the
concentration is greater on the left than the right (remember the
cell membrane controls what small molecules or ions can be transported in
and out of cells).
Therefore for the green particles the diffusion gradient is from left to right and
there is a net movement of the green particles (smaller
molecules) from the left higher concentration to the right lower
concentration passing through the cell membrane in the process.
Also bare in mind that the larger the surface area of a membrane,
the faster the net rate of diffusion of a particular molecule or ion.
Diffusion and
the process of
respiration.
The thin cell membranes allow the diffusion
of small molecules in and out of cells.
Since the capillaries are thin and numerous,
the diffusion distance from cells is short, so transfer of nutrients in, and
waste products out, is as efficient as possible.
As the cells respire they use up
oxygen/glucose, so their concentration falls in the cell. Therefore the
external concentrations (e.g. in capillaries) is higher, so more
oxygen/glucose will diffuse into the cell.
At the same time, the concentration of the
waste product carbon dioxide builds in the cell, and so carbon dioxide will
then naturally diffuse out of the cell to the lower concentration region in
the capillaries.
For more details on gas exchange and other transfer systems
involving diffusion see notes on:
Surface exchange of substances in
animal organisms
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